1. Field of the Invention
This invention relates to a magneto-optical recording medium to be used for high density magnetic recording and reproduction thereof by the use of a magneto-optic effect.
2. Description of the Prior Art
Recently, in the field of the high density recording of information, a magneto-optical recording medium having erasability or rewritability by the use of the magneto-optic effect has become promising, as compared with fixed or non-erasable type information recording such as by an optical disc.
Conventionally, the reading-out for the magneto-optical recording medium mentioned above has been performed by a reading system such as shown in FIG. 1. In FIG. 1A, denoted by 1 is a light beam; denoted by 2 is a polarizing plate; denoted by 3 is a polarizing beam splitter; denoted by 4 is an objective lens; denoted by 5 is a magnetic film; denoted by 6 is an analyzer; denoted by 7 is a condensing lens; denoted by 8 is a photodetector; and denoted by 9 is a substrate. The magnetic film 5 is formed on the substrate 9 made of a glass, resin or other material, by a process such as sputtering or the like and provides a magneto-optical recording medium in which information is recorded as the changes in the direction of magnetization (for example, upwardly or downwardly), by using, e.g., a method of projecting a laser light beam modulated in accordance with information signals.
The beam 1 is linearly polarized by the polarizing plate 2 and projected onto the magnetic beam 5 through the polarizing beam splitter 3 and the objective lens 4. In accordance with the direction of magnetization of the magnetic film 5, as discussed hereinbelow, the plane of polarization of the beam 1 is rotated in the opposite directions owing to the magneto-optic Kerr effect, and the beam is reflected from the magnetic film 5. If, for example, the plane of polarization of the beam reflected by the downwardly magnetized section is rotated through an angle .theta.k, the plane of polarization of the beam reflected by the upwardly magnetized section is rotated through an angle -.theta.k.
If the incident beam is P-polarized light as shown in FIG. 1B, and when the polarized light transmitting direction of the analyzer 6 is disposed in a direction (Q-direction) perpendicular to the above-mentioned polarizing direction -.theta.k, the reflected light from the upwardly magnetized section is intercepted by the analyzer 6, and the transmission component .DELTA., through the analyzer 6, of the light reflected from the downwardly magnetized section passes through the analyzer 6.
By detecting this transmission component by means of the photodetector 8, the recorded information is read out.
Since, however, the Kerr rotation angle .theta.k owing to the above-described magneto-optic Kerr effect is of the order of 0.1.degree., the aforesaid transmission component is extremely minute and thus the S/N ratio (Signal-to-Noise ratio) of the reproduction signal detected by the photodetector is so low level that is insufficient particularly in the case of reading of high frequency signals.
In order to overcome such problem, it has already been proposed in, e.g., Japanese Patent Application Laid-Open No. 156943/1981 that a high refractive index thin film layer is provided on the magnetic film and the reflection factor of the magnetic film is made lower, improving the S/N ratio by use of the Kerr enhancement effect by which the Kerr rotation angle .theta.k is apparently increased.
In such conventional magneto-optical recording medium, however, the reflection factor of the magnetic film can not be set at a constant value because of, e.g., the fact that the range of reflection factor determined by the material, thickness and etc. of the high refractive index thin film layer is restricted. Further, since the setting of the reflection factor is made without paying attention to noises or the like of the photodetector in the reading system described hereinbefore, a maximum reproduction S/N ratio can not always be obtained, particularly, in the case of using a photodetector having a multiplying function.